test+admin login issue

This is branched off the forgot_password because the helper for that is needed here too.

The idea behind this fix is easy: if someone logs in with CAS to Canvas but their user doesn't exist in Canvas, shoot them back to the main site. We know their user exists there, so it can give them a relevant screen for themselves.

Companion PR: https://github.com/beyond-z/beyondz-platform/pull/250
What is the reason for still hard-coding the path?  With two different servers in play, I feel like using config to set the interdependent paths is better.  I think the likelihood of changing the paths is high and needing to do a code deploy to fix it just opens ourselves up to make mistakes (e.g. we have a bunch of changes in progress on staging and we accidentally push those changes in the prod release when trying to update the path since we're racing against time with broken features that users can see).